High-Reflectivity Mg/Al Ohmic Contacts on n-GaN

This work reports a Mg (130 nm)/Al (50 nm) bilayer to realize high-reflectivity ohmic contact for n-GaN. The contact resistivity of 3.75 × 10 ^-4 Ω·cm ² with Mg/Al contact was achieved after annealing at 250 °C in ambient Ar for 1 min. The specific contact resistivity was found to change moderately when the annealing temperature was below 400 °C, but the ohmic characteristics deteriorated significantly when the annealing temperature was above 450 °C. X-ray photoemission spectroscopy revealed that the Ga 3d (Ga-N) peak decreased by 0.28 eV with the increase of the annealing temperature from 250 to 450 °C, which can increase the metal-semiconductor contact barrier height, leading to deteriorated current-voltage characteristics. The reflectivity of the samples was found to decrease with the increase of the annealing temperature, and decreased greatly when the temperature exceeded 350 °C. When the annealing temperature is below 300 °C, the reflectivities of the samples are greater than 90% for wavelengths from 350 to 550 nm. Mg/Al is therefore a promising candidate to serve as a reflective n-GaN electrode for GaN-based flip-chip LEDs to improve the light extraction efficiency.